1. Field of the Invention
The present invention relates to sound control units, particularly to piezoelectric buzzer driving circuits.
2. Description of Related Art
General consumer electronic products, such as personal computers, automobiles, communication terminals etc. can produce sounds, such as warning or ringing sounds. This is done by using internal sound producers, such as buzzers.
One type of buzzer is a piezoelectric buzzer. Referring to FIG. 5, a typical piezoelectric buzzer driving circuit is shown. The piezoelectric buzzer driving circuit includes a driving circuit 10 for driving a piezoelectric buzzer 111. The driving circuit 10 includes a transistor Q10, and two resistors R1 and R2.
The base of the transistor Q10 is connected to a controlling terminal 12, which provides a controlling signal to the piezoelectric buzzer 111 via the resistor R2. The collector of the transistor Q10 is connected to a power supply terminal 11 via the resistor R1. The emitter of the transistor Q10 is grounded.
Two terminals of the resistor R1 are set respectively as a first output terminal 14 and a second output terminal 16 of the driving circuit 10. The piezoelectric buzzer 111 is connected to the first output terminal 14 and the second output terminal 16.
The power supply terminal 11 provides a direct-current power supply with a voltage U11. The controlling signal provided from the controlling terminal 12 is a periodic impulse signal with a constant frequency for controlling the input voltage at the power supply 11. The transistor Q10 may be turned on or off according to the controlling signal.
A voltage across the piezoelectric buzzer 111 is ΔU1, a voltage at the first output terminal 14 is U14, and a voltage at the second output terminal 16 is U16. Therefore, ΔU1 is expressed as: ΔU1=U14−U16.
When the transistor Q10 is turned on, the voltage U14 of the first output terminal 14 is approximately equal to the voltage U11, and the voltage U16 of the second output terminal 16 is approximately equal to zero. Therefore, the voltageΔU1=U14−U16=U11.
When the transistor is turned off, the voltage U14 of the first output terminal 14 and the voltage U16 of the second output terminal 16 are both approximately equal to the voltage U11. Therefore, the voltageΔU1=U14−U16=0.
With the above description, during each on-off period of the transistor Q10, a varied voltage ΔU12 across the piezoelectric buzzer 111 is expressed as ΔU12=U110=U11. That is to say, maximum varied voltage ΔU12 across the piezoelectric buzzer 111 during each on-off period of the transistor Q10 is approximately equal to the voltage U11. On the other hand, a sound efficiency (i.e., electrical energy input against sound energy output) of the piezoelectric buzzer 111 is dependent on the voltage from the power supply terminal 11. The higher the voltage from the power supply terminal 11 is, the better the sound efficiency of the piezoelectric buzzer 111 is. However, high voltage electricity may cause damage in some consumer electronic products.
What is needed, therefore, is to provide a piezoelectric buzzer driving circuit with satisfactory sound efficiency even under low voltage.